Air bubbles in return line at lo-speed pump.

[mgianzero]

I have a 2-speed pump for my system circulation and it is shares the spa jets (4 jets) with another spa-only pump circuit (other 6 jets). At lo-speed (50 GPM) I noticed considerable air bubbles coming from my pool return lines. This is the funny thing. It doesn't seem to do this at hi-speed (105 GPM). Perhaps the water surface is agitated too much to see it but I doubt it. However, when I turn OFF my spa spillway (I have 4 jets going to hot tub on a spillway back to the pool), the bubbles go away on lo-speed. On high speed it makes no difference since there were never bubbles there in the first place.

What sort of air leak would ONLY show itself at a lower pump flow and not high pump flow? I know the spillway is using the spa as a return which uses venturi effect to draw air into the spa jets, but how would this draw the air all the way back to the pool and only at lower flows?

 

[Guest]

not so much likely an air leak as it is cavitation caused by too much head pressure. Spa jets have a lot of head pressure compared to pool returns (which is why spas need more powerful pumps than pools). The fact that it stops when you use high speed or turn off the spillover on low speed pretty much confirms this.
Solution is to only run the spillover and/or spa on high speed.

 

[mas985]

Cavitation is unlikely in this case because it is usually caused by too much suction vaporizing the water. Also, cavitation only occurs in the impeller and the vapor bubbles collapse before exiting the pump which is what causes the damage so you should not see any air in the return lines with true cavitation.

If I read your post correctly, the bubbles occur only when water is returned to both the pool and the spa 4 jets and only at low speed. But the bubbles only come out of the pool returns and not the spa jets? The spa jets I would assume always have bubbles coming out.

Is the spa above, below or at the same level of the pool?

Also, is there air collecting in the filter? If not, then the air may be entering further done the line via the venturis.

This might have to do with how the spa jets are shared with the pool and with the other spa jets on another loop. Are the other 6 jets completely isolated for the 4 jets share with the pool? Do you know where the 4 jets are tied in with the pool plumbing?

Air may be entering through the venturis and somehow getting back to the pool returns. This seems unlikely although depending on how the plumber "shared" the return, it might be possible at low flow rates. Can you post a plumbing diagram?

 

[mgianzero]

Cavitation is unlikely in this case because it is usually caused by too much suction vaporizing the water. Also, cavitation only occurs in the impeller and the vapor bubbles collapse before exiting the pump which is what causes the damage so you should not see any air in the return lines with true cavitation.

If I read your post correctly, the bubbles occur only when water is returned to both the pool and the spa 4 jets and only at low speed. But the bubbles only come out of the pool returns and not the spa jets? The spa jets I would assume always have bubbles coming out.

Is the spa above, below or at the same level of the pool?

Also, is there air collecting in the filter? If not, then the air may be entering further done the line via the venturis.

This might have to do with how the spa jets are shared with the pool and with the other spa jets on another loop. Are the other 6 jets completely isolated for the 4 jets share with the pool? Do you know where the 4 jets are tied in with the pool plumbing?

Air may be entering through the venturis and somehow getting back to the pool returns. This seems unlikely although depending on how the plumber "shared" the return, it might be possible at low flow rates. Can you post a plumbing diagram?

Yes, I agree with you. This certainly cannot be due to cavitation. Cavitation occurs when the pump is starved for water, such as when using an oversized pump or high suction head pressure. This didn't occur at higher flows which should rule these arguments out.

Yes, the bubbles ALSO come out of the spa jets (only 4 that are part of this spillway circuit). However, at low flow they seem to sputter out (clumps of air) and really only out of the two nearest facing jets (closest to the pump). I also figured it was the venturis, however when I cap the open venturi, it doesn't seem to stop any of the air anywhere. Perhaps it's too difficult to slip a 2" cap on it and make it air tight?

There does seem to be just a little air in the pump basket (small bubbles) but they go away at higher flow. I can also burp the filter of a little air. When the pump is high or off, no air seems to be in the pump basket window.

The spa sits 18" above the pool. The spa only shares these 4 jets with the system pump and are the same jets activated for the spa spillover affect. The other 6 jets are on their own circuit. The spa spillway has a hand valve and check valve connecting the pool to the spa. I can actually see the air bubbles in this check valve window swirling around. They go away at hi flow.

Here's a pic of my spillover connection with the check valve that has the air bubbles. Spa return in on the left, pool return is on the right.

I am not sure how the venturis are plumbed into the spa circulation. I guess I'd have to dig it up (I could since there's only dirt) and see where/how it's connected.

 

[mas985]

Normally, air in the returns comes from a suction side leak which then passes the air to the returns. If you have air in the pump basket, then this is likely the source but it should get worst at higher speed and flow rates. So unless you see bubbles on high speed, I don't think that is the problem.

The only other cause I can think of, is that the air is entering via the venturi vent pipe and somehow making it back to the pad. However, in order for this to happen, the spa must be at lower elevation than the pad so air can travel up the jet pipe. Also, the flow rate would have to be low enough so that the air would not be pushed out by the water. Seems unlikely but I guess possible. Kind of a perfect storm in plumbing.

Other than that, I can't think of anything else that would fit the symptoms. You might try a better seal on the vent pipe to see if you can actually shut off all the air. A little plumbers grease on the cap might help seal it. Just make sure that you run the pump on high first to push out all the air.

 

[spishex]

The pump could be pulling air through the strainer lid. On high speed the suction force on the lid would be stronger, pulling it down and sealing the o-ring tighter. Try lubricating the ring and tightening the lid.

 

[mas985]

The pump could be pulling air through the strainer lid. On high speed the suction force on the lid would be stronger, pulling it down and sealing the o-ring tighter. Try lubricating the ring and tightening the lid.

Great suggestion and likely the cause.